A binary ("bilevel") level pixel printer produces an image using dots that are either full on or full off. So long as a bilevel printer is employed to print only text or line images, neither a host processor nor the printer need consider how best to reproduce a gray-scale image. If, however, the host processor is instructed to cause the bilevel printer to reproduce a multi-bit-per-pixel (color or gray scale) raster image, then a conversion of the multi-bit-per-pixel image to a bilevel image must be performed, either in the host processor or in the printer, to enable the image's reproduction on a media sheet. Some common sources of multi-bit-per-pixel image data are scanned pictures, color and gray scale drawings from software illustration packages and patterns generated by such software illustration packages.
Multi-bit-per-pixel image data includes two distinct parts: an image header and the image data. The image header contains information about the image data, such as its height, width, the number of bits-per-pixel and sometimes an image color palette. The image color palette is a table of red, green and blue (RGB) values which define the full range of primary colors available for use in the reproduction of the image. The image data includes the actual multi-bit-per-pixel data that defines the image. Such image data is stored as offsets into the color palette table.
Many prior art systems entirely process an image in the host processor and then transmit it for printing to an attached bilevel printer. The multi-bit-per-pixel image is prepared in the host processor; converted (if necessary) from color to gray levels, scaled to the size required by the connected printer, half-toned to a bilevel bit map, and then compressed and transmitted over an input/output port to the connected printer. Once in the printer, the image is decompressed, processed for printing and printed.
If the multi-bit-per-pixel image created in the host processor is the same "size" (i.e. number of bytes) as the bilevel image required for printing in the printer, conversion of the multi-bit-per-pixel image to a bilevel bit map can occur in either the host processor or the printer without affecting image transmission time between the host and printer. However, many host processors today configure their images based upon a 640 pixel wide by 480 pixel long display presentation. This equates to a 75 pixel per inch image resolution. By contrast, many bilevel image printers today operate at a 600 pixel per inch resolution level. As a result, a 75 multi-bit pixel per inch image stored by the host processor must be converted in either the host or the printer to 600 pixel per inch bilevel pixels for use in the printer. If image conversion takes place in the host processor, the large amount of binary data can create a substantial print delay while the image is being transmitted between the host processor and the printer.
An example will help in understanding this situation. Assume that the host processor assembles a 2 inch by 2 inch image at a 75 pixels per inch resolution. Further assume that there are 8 bits of gray value data per pixel. Thus, the 2 inch by 2 inch host image is represented by 22,500 bytes of binary data. When that host image is converted in the host to a bilevel pixel image for transmission to a bilevel printer, 180,000 bytes of binary data are required to represent the 2 inch by 2 inch image (at 600 pixels per inch). Considering that data is transmitted from a host processor to an attached printer at an approximate rate of 10,000 bytes per second, the transmission time of the bilevel image to the connected printer is approximately 14 seconds. Clearly, if the image is a full size image (e.g. 8".times.11"), the time required for transmission is proportionately greater. The result is that a substantial delay is incurred to enable transmission of bilevel pixel image data between the host processor and the printer.
Accordingly, it is an object of this invention to provide an improved apparatus for processing and displaying bilevel image data wherein printer delays due to image transmission times are minimized.
It is another object of this invention to provide apparatus for processing and displaying bilevel image data which enables processing of multi-bit-per-pixel image data into bilevel image data at a location which achieves minimum image transmission times between a host processor and a connected printer.